The present invention relates to an image forming apparatus which forms a toner image with an electro-photographic method, to a clear layer forming apparatus which forms a clear layer on said toner image, and to an image forming system equipped with the image forming apparatus and the clear layer forming apparatus.
In an image forming apparatus of an electro-photographic method, a toner image is formed on the image carrier of a rotating photoreceptor drum or a photoreceptor belt, and the formed toner image is transferred onto a recording sheet directly or indirectly. Furthermore, fixation is carried out, and forming of an image is thus performed.
For the image forming, the toner image transferred electrostatically onto the recording sheet is fixed into a stabilized state on the recording sheet with heat and pressure by a fixing roller.
FIGS. 5a, 5b, and 5c are schematic cross-sectional views showing the appearances of a fixing roller and a recording sheet.
FIG. 5a shows that fixation begins to be applied by a fixing roller to the recording sheet onto which the toner image has been transferred. Moreover, FIG. 5b shows a state where fixation is being applied by the fixing roller to the recording sheet onto which the toner image has been transferred, and where the fixing roller has completed one revolution after it begins its fixation. Furthermore, FIG. 5c shows a state where fixation is being applied or a state at the end of fixation by the fixing roller onto the recording sheet onto which the toner image has been transferred, at the time of a state where the fixing roller has exceeded one revolution after it begins fixation.
In this case, although it depends on the sizes of recording sheets in the auxiliary scanning direction, if it is not a small recording sheet, a fixing roller may start the second revolution on the recording sheet.
In this case, although a high-temperature heater is equipped in the fixing roller, since heat is taken by the recording sheet during the fixation, if a fixing roller has started the second revolution, the fixing temperature is reduced as shown in the figure (a) of FIG. 6.
Thus, when the fixing temperature is reduce, the surface state of the toner which is fixed on a recording sheet changes. As shown in the figure (b) of FIG. 6, and FIG. 7, the phenomenon in which glossiness is slightly reduced occurs. It will seldom become a problem if this phenomenon occurs on a text document, but it may become a problem during formation of images, such as graphical data and an image data.
In addition, a proposal is made by Unexamined Japanese Patent Application Publication No. 11-2918 and others about an image forming apparatus equipped with this type of fixing device.
In Unexamined Japanese Patent Application Publication No. 11-2918, although the proposal about the improvement of a fixing device is made, the glossiness difference of the toner image based on temperature reduction of the fixing roller mentioned above is not considered at all.